A spray nozzle assembly of the foregoing type is shown and described in U.S. Pat. No. 5,921,472, the disclosure of which is incorporated by reference. Such spray nozzle assemblies typically include a nozzle body which defines a mixing chamber into which a liquid hydrocarbon and pressurized gas, such as steam, are introduced and within which the liquid hydrocarbon is atomized. To enhance liquid atomization within the mixing chamber, an impingement pin extends into the chamber and defines liquid impingement surface on the center line of the mixing chamber in diametrically opposed relation to the liquid inlet against which a pressurized liquid stream impinges and is transversely dispersed and across which pressurized steam from a gas inlet is directed for further interaction and shearing of the liquid into fine droplets. The atomized liquid within the mixing chamber is directed under the force of the pressurized steam through an elongated tubular barrel, commonly disposed within a wall of the catalytic reactor riser, for discharge from a spray tip at a downstream end thereof within the riser. Notwithstanding passage through the elongated tubular barrel the liquid must discharge as a very fine liquid particle spray for optimum performance. To efficiently breakup and transmit the liquid hydrocarbon, the steam cross flow must be at a high volume and pressure, approximately 110 psi, and the liquid pressure must be kept at approximately the same or greater pressure.
In such spray nozzle assemblies, the liquid hydrocarbon flow stream must pass through half the diameter of the mixing chamber before it impacts the impingement pin. Particularly in spray nozzle assemblies with relatively large diameter mixing chambers, such as those having a mixing chamber of four inches and more in diameter, there can be a tendency for the liquid hydrocarbon flow stream introduced into the mixing chamber to only partially impact the impingement surface of the impingement pin. The reason for this is that the liquid flow stream must pass a significant distance through the mixing chamber where it is subjected to a heavy cross flow of steam before impacting the impingement surface. This tends to cause a shift in the liquid flow stream away from the center of the impingement surface, the magnitude of which is dependent upon the velocities of the pressurized steam and liquid flow streams for a particular setup. The shift prevents a portion of the liquid hydrocarbon flow stream from being shattered against the impingement pin, resulting in a significant increase in droplet size for a portion of the spray volume that adversely affects the spray performance. In order to overcome such shift in the liquid flow stream introduced into the mixing chamber, heretofore it has been necessary to increase the liquid pressure even more to overcome the effect of the steam cross flow. This necessitates the need for larger and higher pressure process pumps that are more expensive to operate and more susceptible to breakdowns. On the other hand, operation of such spray nozzles at lower pressures significantly effects spray performance and can create clogging, particularly when spraying heavier crude oils such as resids and petroleum bottoms.